A slight rise in temperature may increase the risk of malaria to hundreds of thousands, if not millions, of people, in areas that are currently too cold for malaria parasites to complete their development, according to a study.
Researchers at Penn State in the US used two of the most important in the world -- Anopheles stephensi and Anopheles gambiae -- to conduct their experiments.
They maintained these malaria-infected mosquitoes in the laboratory under a variety of temperatures ranging from 16 to 20 degrees Celsius.
For the study, published in the journal Biology Letters, the researchers maintained a separate control set of mosquitoes at 27 degrees Celsius, which is the temperature at which malaria transmission is typically highest.
"Our work shows that even small increases in temperature could dramatically increase malaria infections in humans because the parasites develop much faster at these lower temperatures than has been previously estimated," said Jessica Waite, senior scientist at Penn State.
"Parasite development rate further increases when temperatures fluctuate naturally, from cooler at night to warmer in the day," she said.
In addition, the team varied the daily temperatures by 10 degrees Celsius -- 5 degrees Celsius above and below the daily mean -- since such variation in temperature is common in natural settings when it is cooler at night and warmer in the daytime.
The traditional model estimates that parasites in the mosquito take 56 days to develop at temperatures just above the minimum threshold for development -- a cool 18 degrees Celsius, or 64 degrees Fahrenheit.
However, the study shows that as few as 31 days are required for such development for Anopheles stephensi.
The researchers also found that variation in temperature at this cooler end of the range promotes faster parasite development. Parasites developed in as few as 27 days at 18 degrees Celsius, or 64 degrees Fahrenheit, under realistic variable temperature conditions.
"The rate of malaria transmission to humans is strongly determined by the time it takes for the parasites to develop in the mosquito," said Matthew Thomas, professor and Huck scholar in ecological entomology at Penn State.
"The quicker the parasites develop, the greater the chance that the mosquito will survive long enough for the parasites to complete their development and be transmitted to humans," he added.
According to Waite, the findings have implications for potentially millions of people living in the higher elevations of Africa, such as the Kenyan and Ethiopian highlands, and in South America.
"As temperatures increase with climate change, infectious mosquitoes in areas surrounding mountains, for example, may be able to transmit the parasite higher up the mountains than they have in the past," she said.
"Our results suggest that small rises in temperature could lead to greater increases in transmission risk than previously thought," she added.